Field of the Invention
The present invention relates to a shift register and, more particularly, to a shift register capable of achieving an enhancement in drivability through prevention of leakage current, and reducing bezel size through reduction in the falling edge time of a scan pulse.
Discussion of the Related Art
In general, a liquid crystal display (LCD) device is adapted to display an image by adjusting light transmittance of a liquid crystal using an electric field. To this end, the LCD device includes a liquid crystal panel having pixel areas arranged in matrix form, and a driving circuit for driving the liquid crystal panel.
In the liquid crystal panel, a plurality of gate lines and a plurality of data lines are arranged to cross each other, and the pixel areas are defined respectively at intersections of the gate lines and the data lines. Also formed in the liquid crystal panel are pixel electrodes and a common electrode for applying the electric field to the respective pixel areas.
Each of the pixel electrodes is connected to an associated one of the data lines via the source terminal and drain terminal of a thin film transistor (TFT), which is a switching device. The TFT is turned on in response to a scan pulse applied to the gate terminal thereof via an associated one of the gate lines, so as to charge a data signal on the associated data line to a pixel voltage.
The driving circuit includes a gate driver for driving the gate lines, a data driver for driving the data lines, a timing controller for supplying control signals for control of the gate driver and data driver, and a power supply for supplying various drive voltages to be used in the LCD device.
The gate driver sequentially supplies scan pulses to the gate lines to sequentially drive liquid crystal cells in the liquid crystal panel on a line-by-line basis. In order to sequentially output the above-mentioned scan pulses, the gate driver includes a shift register.
In a conventional case, the shift register includes a plurality of stages each having a plurality of switching elements.
Each stage includes a pull-up switching element for outputting a scan pulse. That is, the pull-up switching element outputs, in an ON state thereof, a clock pulse input received from the outside of the driving circuit, as scan pulses Vout1 to Vout3. In the conventional case, a single discharge voltage is used and, as such, leakage current generated at the pull-up switching element in a discharged state (disable state) of the set node in the stage is large. Such leakage current drops the high-level voltage value of the scan pulse output from the stage when the stage is in an enable state, thereby reducing drivability of the shift register. As a result, degradation of picture quality occurs in an image display apparatus.